In line with the fact that the size of a semiconductor device has been gradually reduced and the number of layers of metal interconnections has been gradually increased, surface irregularities in each layer are transferred to the next layer, and thus, a curvature of the lowermost surface becomes important. The curvature may have a significantly effect to such a degree that it may be difficult to perform a photolithography process in the next step. Thus, in order to improve the yield of the semiconductor device, a planarization process for removing the curvature of irregular surfaces, which occur in various process steps, is essentially used. There are various planarization methods such as a method of reflowing after the formation of a thin film, a method of etching back after the formation of a thin film, and a method of chemical mechanical polishing (CMP).
A chemical mechanical polishing process denotes a process of smoothly polishing a surface of a semiconductor wafer by providing a slurry containing an abrasive and various compounds while the surface of the semiconductor wafer is in contact with a polishing pad and is subjected to a rotational motion. That is, the chemical mechanical polishing process denotes that a substrate or a surface of a top layer of the substrate is planarized by being chemically and mechanically polished by the slurry and the polishing pad. In general, it is known that a process of forming a metal oxide (MOx) by an oxidizer and a process of removing the formed metal oxide with an abrasive repeatedly occur in a metal polishing process.
A polishing process of a tungsten layer, which is widely used as an interconnection of the semiconductor device, is also performed by a mechanism in which a process of forming a tungsten oxide (WO3) by an oxidizer and a potential control agent and a process of removing the tungsten oxide with an abrasive are repeated. Also, an insulation layer or a pattern, such as a trench, may be formed under the tungsten layer. In this case, a high polishing selectivity between the tungsten layer and the insulation layer is required in the polishing process. That is, a slurry, which does not polish the insulation layer while well polishing the tungsten layer, is required.
Thus, in order to improve the polishing selectivity of tungsten to the insulation layer, various components have been added to the slurry or the amounts of the oxidizer and catalyst contained in the slurry have been controlled. However, in spite of these efforts, a slurry for polishing tungsten, which exhibits a high polishing selectivity, has not been developed so far.
In Korean Patent Registration No. 10-0948814, a slurry for polishing tungsten is proposed in which polishing is performed in two steps, but, in this case, the process may be complicated and productivity may be reduced.